This invention relates to electrostatography and more particularly to an apparatus and method of developing latent image nonuniform charge patterns on an imaging member.
Xerography as originally described in U.S. Pat. No. 2,297,691 to Carlson and later related patents generally includes the steps of charging a photoconductive insulating member to sensitize it and then subjecting the photoconductive member to a light image or other pattern of activating electromagnetic radiation which serves to dissipate charge in radiation struck areas, thus leaving a charge pattern or latent electrostatic image on the photoconductor conforming to the radiation pattern. This radiation pattern is generally referred to as a uniform or a homogeneous charge pattern because there is substantially no potential gradient between small discrete areas where the charge has not been dissipated by exposure to the actinic radiation. Thus, in areas having a uniform charge thereon the lines of force lie within the photoconductive material and do not extend out from the exposed surface of the photoconductive material except at the very edge of an imaging area where a discharged portion of the photoreceptor is adjacent to a charged portion. In this area, the lines of force are present in what has been referred to as a fringe field and extend above the exposed surface of the photoconductive imaging member.
While this uniform charge pattern in unexposed areas was difficult to develop in solid dark image areas, because of the nature of the field present, various techniques were employed. One particularly satifactory technique uses a development electrode, which permits the development of the solid large areas with the customary development materials, i.e. a two component developer including a carrier material and a toner material. In these systems the toner is triboelectrically charged by contact with the carrier particles, the charge being opposite in sign to that present on the photoconductive imaging member. A disadvantage of this type of development system is commonly referred to as "the starvation effect" because as large quantities of the toner material are deposited in image configuration the ratio of the toner to the carrier present in the developer mixture changes thus requiring constant addition of toner to prevent the depletion thereof accompanied by incomplete deposition in the reproduction subsequent in time.
U.S. Pat. Nos. 3,234,017 and 3,519,461 disclose techniques whereby a nonuniform or nonhomogeneous charge pattern in image configuration is produced on an imaging surface wherein small individual, discrete surface elements adjacent to each other within the imaging area are either oppositely charged or some discrete areas are charged and the adjacent areas discharged in order establish a potential gradient between adjacent discrete areas thus permitting the development thereof by utilizing polarizable minute uncharged toner particles. These particles are polarized in the fringe fields protruding above the surface of the imaging member and are thus attracted in image configuration to the imaging member. This system will, because of its nature, develop solid areas without the necessity for development electrodes as indicated above where uniform fields and two component developers are employed. In addition, no starvation effects are present because the developer is 100 percent toner.
Unfortunately, because of the nature of this system, several disadvantages are present which create problems in the handling and development of the latent image using single component uncharged toner particles. The strength of the fringe fields employed, that is, the forces which attract and hold the toner particles in image configuration to the imaging member are not as great as those present in electrostatographic systems wherein uniform charge patterns are present on the imaging member and the particles are attracted as a result of triboelectricity. Thus, it has been found that the raking action of a brush, even a soft fiber brush or a carrierless magnetic brush formed of magnetic toner only, utilizes forces greater than present in the system for holding the toner to the imaging member. In addition, since the toner particles should remain uncharged in order that they are capable of being polarized by the fringe fields present in the nonuniform charge pattern, care must be taken to prevent charging of the toner particles by contact with materials having a different relative position in the triboelectric series. A development system is required that exerts less force on the toner particles than that due to the nonuniform charge pattern in order to prevent removal of toner from desired image portions. Also, any unwanted charging of the toner particles will produce forces stronger than the forces due to the polarization of the particles in non-uniform fields and will produce unbalanced deposition of the toner particles in both types of discrete areas present within the charged pattern as well as excessive background in non-image areas.
While the advantages relative to the development of nonuniform charge pattern appear to be meritorious, the industry has not developed in this direction most likely because of the large commercial success of electrostatographic devices employing uniform charge patterns together with dual development systems wherein development electrodes of a various nature have been employed in order to permit the development of large dark areas. One extremely useful development device where both single component developer, that is, toner alone, and dual development, that is, where a carrier and a toner are employed, is the magnetic brush. These devices are well documented in the patent literature and are currently probably the most widely used means of developing electrostatographic images. Magnetic brushes have also been employed in single component development wherein the toner contains magnetic pigments. It can thus be seen that in such magnetic brush systems either a magnetic carrier material must be present which is also triboelectrically active with respect to the toner particles involved or else the toner particles themselves must be magnetic in order to be employed with a magnetic brush device. This is a serious handicap where reproductions other than black or brown images are desired since magnetic particles included in the toner particles are extremely dark by nature and make it substantially impossible to develop images in suitable colors other than black or brown. Further, as indicated above, problems are created because of the forces inherent in the magnetic development systems.
Accordingly, it is a primary object of the present invention to provide an improved apparatus and method for developing nonuniform charge patterns on an imaging surface.
______________________________________ PRIOR ART STATEMENT INVENTORS U.S. PATENT ISSUE DATE ______________________________________ Heyl et al 3,234,017 February 8, 1966 Stowell 3,519,461 July 7, 1970 Raschke 4,048,921 September 20, 1977 Bean 4,103,994 August 1, 1979 ______________________________________
Heyl et al U.S. Pat. No. 3,234,017--A non-homogeneous charge pattern is produced by subjecting the photoconductive layer to an electric potential by means of a corona discharge large enough to cause the photoconductive layer to break down electrically at multiple closely spaced discrete locations and depositing uncharged toner to develop the image.
Stowell U.S. Pat. No. 3,519,461--A method of electrostatic printing is disclosed whereby electrical dipoles are established on a dielectric surface and developed with uncharged polarizable toner powder.
Raschke U.S. Pat. No. 4,048,921--An electrostatic charge pattern is established on a dielectric surface and this charge pattern is toned with an insulating fluid containing small particles of high dielectric constant which thereby obtain an induced dipole moment.
Bean U.S. Pat. No. 4,103,994--A recording member is disclosed including a photoconductive layer having embedded therein at least a pair of insulated conductive members. A potential difference is imposed across the pair of conductors to form an electrical field. The imaging member is exposed in image configuration and the latent image developed with uncharged insulating particles using a magnetic development system.